312 SYMBIOTIC NITROGEN FIXATION 



of protein, asparagin, and other soluble forms, is also clear. The 

 presence of various aliphatic and carbocyclic amino-acids has been 

 mentioned." 



Sources of Energy. Under natural conditions the legume bacteria 

 undoubtedly obtain the energy required for the endothermic reaction 

 which they catalyze from the plant carbohydrates. It has long been 

 known that decoctions of the legumes makes the best media on which 

 to grow these organisms. Temple found that the presence of 

 ground alfalfa caused a rapid multiplication of the organisms either 

 in solution or in soil. Grieg-Smith found dextrose, levulose, sac- 

 charose, maltose, and mannite to furnish a good source of carbon for 

 the organisms, but lactose was a very poor nutrient. Temple found 

 saccharose and dextrose superior to lactose, whereas he found levulose 

 wholly unsuited to their needs. 



No one so far has attempted to measure their energy requirements 

 when growing under their natural symbiotic condition. Fred has 

 studied their growth apart from the host plant and found that when 

 considered per unit of carbohydrate consumed the legume bacteria 

 fix as much or more nitrogen than Azotobacter. 



"Aeration. The legume bacteria are all aerobic and the nodules 

 on the roots of the plants are usually near the surface. Although 

 nodules will form on plants grown in water cultures, yet they are 

 not as large and active as when grown in a well aerated soil. The 

 addition of oil to a soil or water culture in which legumes are growing 

 prevents the formation of the nodules. Moreover, as shown by 

 Whiting, the legumes get their nitrogen through the root and not 

 the leaves. The result of cultivation of legumes is, therefore, 

 threefold: (1) The loosening up of the soil, thus making available 

 to the nodule bacteria atmospheric nitrogen and oxygen; (2) the 

 working of the soil increases other bacterial activity wiiich in turn 

 renders soluble potassium, phosphorus, and other essential elements 

 in the soil; (3) the loose aerated surface tends to conserve the 

 moisture of the lower layers which can be drawn on by the plant, 

 thus making more nearly optimum moisture conditions. 



Moisture. The root systems of plants vary greatly with the 

 moisture content of the soil. Gain found that legumes grown in 

 moist soil spread widely, were full of water, became covered with 

 root hairs, and presented a large surface of young tissues. In the 

 dry soil the roots were less spreading and the epidermis was greatly 

 thickened. 



In moist soil the tubercles of the peas were scattered all over the 

 roots, were five or six times as abundant as in the dry soil, and were 

 about four times as large and ovoid in shape; w r hile in the dry soil 

 no tubercles were produced on the superficial roots. At a depth of 

 about 20 centimeters some tubercles were found of a hemispherical 

 shape and much smaller than those grown in moist soil. 



